如图所示，abc是光滑的轨道，其中ab是水平的，bc为与ab相切的位于竖直平面内的半圆，半径R=0.30m．质量m=0.20kg的小球A静止在轨道上，另一质量M=0.60kg、速度V₀=5.5m/s的小球B与小球A正碰．已知相碰后小球A经过半圆的最高点c落到轨道上距b点为，l=4 $\sqrt{2}$ R处，重力加速度g=10m/s²，求：

假设含有一对同源染色体的精原细胞的DNA分子用¹⁵N标记，并供给¹⁴N的原料。该细胞进行减数分裂产生的4个精子中，含¹⁴N的精子所占的比例为（   ）

①得意时，挫折会使我们更清醒，避免盲目乐观、精神懈怠

②挫折能磨炼人的意志

③挫折能增长人的才干和智慧

④失意时，挫折会使我们获得更加丰富的生活经验

【1】Fires can burn down trees. They can hurt people. They are very dangerous. We must be careful with fires. What must we do when there is a fire? We must go outside quickly and never take the lift. We must call 119. We have a fire drill at school every year.

【2】A fire drill is not a real (真实的）fire. It helps us prepare (准备) for a real fire. First, the fire alarm sounds. Next we all line up in the classroom. Then we leave the classroom quickly. We mustn't run. We must leave all our things in the classroom.

【3】Then we walk down the stairs and go outside. We line up in the playground. Our teacher checks on us. We wait for the firemen to come.